Making firefox as default Browser for c#

Please Help me out....Iam banging my head to the wall now just because of this problem....
I want to get the HTML code of a Website using the Class of HttpWebRequst and HttpWebResponse.....
the main problem is that whatever web requests are made...they are done using Internet Explorer.....Please tell me a way that how can I send HttpWebRequests via Mozilla Firefox....

The Exact issue is that for a certain URL the Internet Explorer shows page cannot be displayed because it has a 500 internal server error....same Error code occurs in Mozilla Firefox but it does show some html also....